The present invention relates to the field of hydraulic pumps. More particularly, the invention relates to an apparatus and method for ganging multiple open circuit variable displacement pumps.
Many applications of open circuit variable pumps may require large displacements, but still desire the size advantages (height, width and sometimes length) of smaller displacement units. Conventional multiple open circuit pumps comprise a plurality of smaller pumps ganged together by connecting their output lines into a single combined output line to provide pressurized fluid to a load. Combined multiple pumps offer the advantage of higher filling speeds and lower cost. Two or more units can be combined to operate as a single larger unit, but control of the combined unit has been problematic. Undesirable interactions often occur between the individual units. The functional characteristics typical of an individual unit are generally compromised. An apparatus and a method for combining multiple pumps while maintaining the functional characteristics of the individual units are needed.
Variable displacement open circuit pumps supply a unidirectional flow of pressurized fluid for driving working devices under load, such as hydraulic motors or cylinders. Various mechanisms are incorporated to control the flow and pressure of fluid from the pump in response to varying operating load requirements. One such mechanism is a load sensing control that varies the fluid displacement of the pump in a manner that provides the flow to the working device as determined by the flow command typically set by the system operator.
Another design feature of variable displacement open circuit pumps is the ability to operate in an over-center condition. When operating in the over-center condition, the pump consumes rather than supplies fluid flow. This over-center operating condition serves to accommodate oil that is "stored" in the load circuit during transient flow conditions due to the compression and containment of fluid within the pump output line.
In a conventional multiple ganged pump system, more particularly in a two variable displacement open circuit pump system, an undesirable fluid circulation pattern may occur when one pump operates in the over-center condition and the other pump continues to operate with a positive fluid displacement with its output flow going to the over-center pump. Overall there is no net output flow, however fluid is flowing in a recirculation pattern between the two pumps. This recirculating flow only serves to needlessly waste energy and generate heat in the system.
When ganging multiple variable displacement open circuit pumps for the purpose of combined flow, it is necessary to incorporate modifications to the otherwise standard load sensing control and over-center function to enable the pumps to interact compatibly and function in a manner equivalent to a single large displacement pump.
Therefore, a primary objective of the present invention is the provision of an improved means and method of ganging multiple open circuit pumps so as to provide control functions which closely emulate those of a single pump.
A further objective of this invention is the provision of a control system for multiple ganged open circuit pumps which reduces the problems associated with over-center operation, while maintaining operative stability.
A further objective of this invention is the provision of a gang of open circuit pumps wherein the pumps operate at a no-flow standby condition by maintaining different pressure settings in their respective load sensing controls.
A further objective of this invention is the provision of a gang of open circuit pumps wherein only one pump has over-center capability while all of the remaining pumps that are ganged together have a zero degree stop to prevent them from operating over-center.